A shuttlecock is a projectile used in the sport of Badminton. Traditionally, a shuttlecock is made from a head of cork and a skirt of overlapping goose feathers (See diagram depicting typical yellow nylon shuttlecock).
For most casual players, this has been replaced by a shuttlecock made using a plastic (usually nylon) or rubber head and a plastic (usually nylon) skirt. The feather shuttlecock is still used by "serious" players and for competitions. It should be noted that the feathered variety is more expensive and far more likely to break during a match. You also have to hit it about 3 times as hard.
The shuttlecock is also called a birdie, bird, or shuttle. The shuttle part of its name was probably derived from its back and forth motion, like the shuttle in a loom, and the cock from the feathers of the traditional shuttlecock, like a birds crest.